As for storing units, I would be more interested in being able to store recruits from RCs from the region. You could then have priority in getting those recruits. The units would not have been recruited, they would just be reserved for that person.
That's...way too open to exploitation. Want to screw your own realm over? Just reserve a ton of the best infantry during wartime and never use it. Not to mention rich dukes preventing anyone else from hiring SF, for example.
